Excited Potential Energy Surfaces Research Articles

The role of the ground and excited potential energy surfaces in the O(1D and 3P)+SiH4 reactions: A theoretical study

The O(1D and 3P)+SiH4 reactions have been studied using ab initio/Rice–Ramsperger–Kassel–Marcus calculations to investigate possible formation mechanisms of various products in combustion and chemical vapor deposition processes. The relative branching ratios for various products formed through the O(1D)+SiH4 reaction involving the long-lived H3SiOH intermediate are calculated as 55.5% for the H2SiO/HSiOH+2H channel, 28.4% for the SiO+2H2 channel, 9.9% for the OH+SiH3 channel, 3.2% for the H2O+SiH2 channel, and 3.0% for the HSiO/SiOH+H2+H channel. These results significantly differ from those obtained in experiment, implying that the O(1D)+SiH4 reaction can take place through a mechanism other than the insertion mechanism. While the O(3P)+SiH4 reaction takes place by the abstraction mechanism, the O(1D)+SiH4 reaction can occur through both insertion and addition/abstraction mechanisms. The addition/abstraction mechanism occurring on the first excited potential energy surface is demonstrated to provide a significant contribution to the reaction products and to account for the forward scattering of the OH products observed in experiment. Finally, heats of formation for various species involving Si atom are computed employing the Gaussian 3 theory.

Coherent mechanism of robust population inversion

A coherent mechanism of robust population inversion in atomic and molecular systems by a chirped field is presented. It is demonstrated that a field of sufficiently high chirp rate imposes a certain relative phase between a ground and excited state wavefunction of a two-level system. The value of the relative phase angle is thus restricted to be negative and close to 0 or - pi for positive and negative chirp, respectively. This explains the unidirectionality of the population transfer from the ground to the excited state.In a molecular system composed of a ground and excited potential energy surface the symmetry between the action of a pulse with a large positive and negative chirp is broken. The same framwork of the coherent mechanism can explain the symmetry breaking and the population inversion due to a positive chirped field.

VTST kinetics study of the [formula omitted] reactions based on CASSCF and CASPT2 ab initio calculations including excited potential energy surfaces

We have carried out a CASSCF and CASPT2 ab initio study of the N( 2 D)+ O 2 system, that is very relevant in the upper atmosphere. The saddle points of the PESs involved in the two reactions that can occur ( O( 1 D)+ NO (1) and O( 3 P)+ NO (2)) have been characterized. According to TST and VTST calculations performed, the N( 2 D)+ O 2 reactivity takes place through reaction (2). Also, a good agreement with the experimental kinetics data corresponding to the global deactivation of N( 2D) has been obtained. These results suggest that reaction (2) dominates over the physical electronic quenching of N( 2D).

The role of rotation in the calculated ultraviolet photodissociation spectrum of ozone

We present exact quantum calculations of the photodissociation of ozone in the Hartley band. These calculations rely on an hyperspherical description of the system, including rotation. A pseudospectral approach has been used for an efficient implementation of this scheme. The autocorrelation function has been directly computed by means of a Lanczos scheme, augmented by a complex absorbing potential. Using a single excited potential energy surface (D 1B2), calculations up to J=17 are reported. It is shown that in these conditions rotation has only marginal effects over the first 500 fs. The origin of the observed experimental temperature dependence is discussed in this context.

Electronic spectroscopy and excited state dynamics of the Al-H2/D2 complex.

The electronic spectra of the Al-H2 and Al-D2 complexes are investigated in a collaborative experimental and theoretical study. The complexes were prepared in a pulsed supersonic beam and detected with laser fluorescence excitation spectroscopy. Transitions to bound vibrational levels in electronic states correlating with the excited-state Al(3d, 4p, 4d) + H2/D2 asymptotes were observed by monitoring emission from lower excited Al atomic levels, formed in the non-radiative decay of the excited complex. Fluorescence depletion has also been used to verify that the observed Al-H2 bands all involve the same molecular carrier. The bands have been assigned to the more strongly bound Al-oH2 and Al-pD2 nuclear spin modifications. In contrast to our previous observations for Al(5s)-H2 [X. Yang and P. J. Dagdigian, J. Chem. Phys., 1998, 109, 8920], for which only one potential energy surface (PES) emanates from the dissociation asymptote, the Lorentzian widths of the different vibrational bands in the 3d, 4p, 4d<--3p transitions vary widely, in some cases allowing resolution of the rotational structure of the bands. With the help of the calculated Al(3p)-oH2/pD2 dissociation energies, binding energies of the observed excited vibronic levels are reported. The mechanism of predissociation is investigated theoretically through ab initio calculation of C2 nu cuts of the excited PESs. It is concluded that predissociation occurs through coupling with the repulsive Al(4s)-H2 PES. With these calculations, a qualitative interpretation of the observed bands could be made.

Photodissociation of CF2=CFCl at 193 nm by translational spectroscopy

The photodissociation of CF2=CFCl at 193 nm has been studied by product translational spectroscopy. Results show that the molecule undergoes competing dissociation into three pairs of products: (1) CFCl+CF2, (2) C2F2Cl+F, and (3) C2F3+Cl with a relative yield of 1.0:∼0.1:1.4. When photolyzed with polarized laser light, reactions (1) and (3) were detected with fragments recoiling anisotropically, indicating that dissociation occurs primarily on the excited potential energy surface. From the maximum kinetic energy derived for reactions (2) and (3), a upper limit of 110 and 90 kcal/mol was determined for the C–F bond and the C–Cl bond dissociation energy, respectively. In addition to these primary dissociation channels, we have observed the secondary products, CF+CF2 and F+C2F2 from the primary fragment C2F3 by absorbing an additional photon.

Hybrid quantum/classical study of ICN in an Ar matrix: Photofragmentation and cage exit

The à continuum photoexcitation of ICN in an Ar matrix is studied using an implementation of the molecular dynamics with quantum transitions method of Tully. Five excited electronic potential energy surfaces of the ICN molecule, Π0+3, Π11(A′,A″), Π13(A′,A″), as well as its ground state, are included in these calculations. The couplings between electronic states at large I–CN internuclear distances are modeled using a diatomic in molecules treatment of the mixing of the different spin-orbit states of iodine induced by the Ar atoms. The electronic motion, as well as the I–CN distance and the corresponding bending angle, are treated quantum mechanically using wave-packet techniques. The rotation and translation of the ICN molecule in the Ar cage are treated classically, as well as the motion of the Ar atoms. In contrast with previous calculations, in which all nuclear degrees of freedom were treated classically, we found a 2% of CN cage exit during the first 0.5 ps of the dynamics.

Experimental and theoretical study of the electronic spectrum of the BAr2 complex: Transition to the excited valence B(2s2p2 2D) state

The 2s2p2 2D←2s22p 2P valence transition in the BAr2 cluster is investigated in a collaborative experimental and theoretical study. Laser fluorescence excitation spectra of a supersonic expansion of B atoms entrained in Ar at high source backing pressures display several features not assignable to the BAr complex. Resonance fluorescence is not observed, but instead emission from the lower 3s state. Size-selected fluorescence depletion spectra show that these features in the excitation spectrum are primarily due to the BAr2 complex. This electronic transition within BAr2 is modeled theoretically, similarly to our earlier study of the 3s←2p transition [M. H. Alexander et al., J. Chem. Phys. 106, 6320 (1997)]. The excited potential energy surfaces of the fivefold degenerate B(2s2p2 2D) state within the ternary complex are computed in a pairwise–additive model employing diatomic BAr potential energy curves which reproduce our previous experimental observations on the electronic states emanating from the B(2D)+Ar asymptote. The simulated absorption spectrum reproduces reasonably well the observed fluorescence depletion spectrum. The theoretical model lends insight into the energetics of the approach of B to multiple Ar atoms, and how the orientation of B p-orbitals governs the stability of the complex.

The product rovibrational and spin–orbit state dependent dynamics of the complex reaction H+CO2→OH(2Π;ν,N,Ω, f)+CO: Memories of a lifetime

The product-state-resolved dynamics of the reaction H+CO2→OH(2Π;ν,N,Ω,f)+CO have been explored in the gas phase at 298 K and center-of-mass collision energies of 2.5 and 1.8 eV (respectively, 241 and 174 kJ mol−1), using photon initiation coupled with Doppler-resolved laser-induced fluorescence detection. A broad range of quantum-state-resolved differential cross sections (DCSs) and correlated product kinetic energy distributions have been measured to explore their sensitivity to spin–orbit, Λ-doublet, rotational and vibrational state selection in the scattered OH. The new measurements reveal a rich dynamical picture. The channels leading to OH(Ω,N∼1) are remarkably sensitive to the choice of spin–orbit state: Those accessing the lower state, Ω=3/2, display near-symmetric forward–backward DCSs consistent with the intermediacy of a short-lived, rotating HOCO (X̃ 2A′) collision complex, but those accessing the excited spin–orbit state, Ω=1/2, are strongly focused backwards at the higher collision energy, indicating an alternative, near-direct microscopic pathway proceeding via an excited potential energy surface. The new results offer a new way of reconciling the conflicting results of earlier ultrafast kinetic studies. At the higher collision energy, the state-resolved DCSs for the channels leading to OH(Ω,N∼5–11) shift from forward–backward symmetric toward sideways–forward scattering, a behavior which resembles that found for the analogous reaction of fast H atoms with N2O. The correlated product kinetic energy distributions also bear a similarity to the H/N2O reaction; on average, 40% of the available energy is concentrated in rotation and/or vibration in the scattered CO, somewhat less than predicted by a phase space theory calculation. At the lower collision energy the discrepancy is much greater, and the fraction of internal excitation in the CO falls closer to 30%. All the results are consistent with a dynamical model involving short-lived collision complexes with mean lifetimes comparable with or somewhat shorter than their mean rotational periods. The analysis suggests a potential new stereodynamical strategy, “freeze-frame imaging,” through which the “chemical shape” of the target CO2 molecule might be viewed via the measurement of product DCSs in the low temperature environment of a supersonic molecular beam.

Energy gradients and effective density differences in electron propagator theory

Efficient calculations of adiabatic electron binding energies require gradients of ground and excited potential energy surfaces. These surfaces may be inferred from reference-state total energies and vertical electron binding energies of the electron propagator. Reference-state total energies from many-body perturbation theory may be derived from electron propagator theory and gradients of these expressions are already known. The missing information for final-state optimization therefore is provided here. Gradients of electron propagator poles (ionization energies and electron affinities) with respect to nuclear positions in the second-order, 2p–h Tamm–Dancoff and nondiagonal, renormalized, second-order approximations of electron propagator theory are derived. Effective electron density difference matrices corresponding to these poles are by-products of the derivations.

A theoretical study of the dynamics of the O(1D)+HD reaction at 0.196 eV collision energy: comparison with experimental results

Differential cross-sections for the O(1D)+HD reaction have been obtained from quasiclassical trajectory calculations performed on recent ab initio 1A′ and 1A′′ adiabatic potential energy surfaces. The calculations were carried out for the conditions of a recent crossed molecular beam experiment in which differential cross-sections were measured with partial state resolution in the internal states of the OH product. The simulation of the experimental data with the results of the present calculations shows qualitative agreement with some noteworthy discrepancies. The comparison between theory and experiment does not provide any evidence for the contribution of the first excited adiabatic potential energy surface to the reaction.

Ultraviolet photodissociation of furan probed by tunable synchrotron radiation

The photodissociation dynamics of furan at 193 nm have been studied using photofragment translational spectroscopy with tunable vacuum ultraviolet (VUV) probe provided by synchrotron radiation on the Chemical Dynamics Beamline at the Advanced Light Source. Three primary channels are observed: HCO+C3H3, CO+C3H4, and H2CCO+C2H2. The evidence suggests that the two closed-shell channels occur on the ground-state potential energy surface (PES) following internal conversion, while the radical channel likely takes place on an excited PES. All channels exhibit a barrier for dissociation with the acetylene+ketene channel having the largest value at about 25 kcal/mol. Angular distribution measurements show anisotropy only for the radical channel. These findings are consistent with a rapid excited state dissociation for the radical channel and slow dissociation for the other two pathways. The two ground-state dissociation channels—propyne+CO and acetylene+ketene—should be important in the thermal decomposition of furan as was found in pyrolytic studies [A. Lifshitz, M. Bidani, and S. Bidani, J. Phys. Chem. 90, 5373 (1986)] and theoretical investigations [R. Liu, X. Zhou, and L. Zhai, J. Comput. Chem. 19, 240 (1998)].

Spectroscopic determination of ground and excited state vibrational potential energy surfaces

Far-infrared spectra, mid-infrared combination band spectra, Raman spectra, and dispersed fluorescence spectra of non-rigid molecules can be used to determine the energies of many of the quantum states of conformationally important vibrations such as out-of-plane ring modes, internal rotations, and molecular inversions in their ground electronic states. Similarly, the fluorescence excitation spectra of jet-cooled molecules, together with electronic absorption spectra, provide the information for determining the vibronic energy levels of electronic excited states. One- or two-dimensional potential energy functions, which govern the conformational changes along the vibrational coordinates, can be determined from these types of data for selected molecules. From these functions the molecular structures, the relative energies between different conformations, the barriers to molecular interconversions, and the forces responsible for the structures can be ascertained. This review describes the experimental and theoretical methodology for carrying out the potential energy determinations and presents a summary of work that has been carried out for both electronic ground and excited states. The results for the out-of-plane ring motions of four-, five-, and six-membered rings will be presented, and results for several molecules with unusual properties will be cited. Potential energy functions for the carbonyl wagging and ring modes for several cyclic ketones in their S1(n,pi*) states will also be discussed. Potential energy surfaces for the three internal rotations, including the one governing the photoisomerization process, will be examined for trans-stilbene in both its S0 and S1(pi,pi*) states. For the bicyclic molecules in the indan family, the two-dimensional potential energy surfaces for the highly interacting ring-puckering and ring-flapping motions in both the S0 and S1(pi,pi*) states have also been determined using all of the spectroscopic methods mentioned above. Here, the effect of the electronic transition on the potential energy surface and hence the molecular structure can be ascertained.

Solvation of clyclopentadienyl and substituted clyclopentadienyl radicals in small clusters. III. Pre-reactive clusters

The hydrogen abstraction reaction between cyclopentadienyl radicals [Xcpd, X=H, CH3(m), F, CN] and substituted methanes (CH4, C2H6, CH3CH2OH, CH3Cl, CH2F2, CHF3, and CH3OH) is studied for the isolated one-to-one van der Waals clusters created in a supersonic expansion. Three different types of fluorescence excitation spectra are characterized for these cluster systems: (1) sharp spectra are observed for some clusters, suggesting no cluster chemistry for either the ground or excited electronic states of Xcpd—CNcpd/CH3Cl, CH2F2, CHF3, CH3OH; (2) broad spectra are observed suggesting initiation of cluster chemistry on the excited state cluster potential energy surface—CNcpd–CH4, Fcpd–CHF2Cl, CHF3; and (3) only a greatly reduced bare radical signal is observed, but no cluster emission can be detected—cpd, mcpd/all substituted methanes, Fcpd–CH2F2, CH3Cl, CH3CH2OH, CH3OH, C2H6, and CNcpd/C2H6, CH3CH2OH. These results, taken together, suggest that the Xcpd radicals undergo an excited electronic state electrophilic hydrogen abstraction reaction with substituted methanes. The radical reactivities are in the order mcpd∼cpd&amp;gt;Fcpd&amp;gt;CNcpd and the substituted methane reactivities are in the order C2H6&amp;gt;C2H5OH&amp;gt;CH4&amp;gt;CH3Cl∼CH3OH&amp;gt;CH2F2&amp;gt;CHF2Cl&amp;gt;CHF3. All Xcpd radicals show intense, sharp spectra with CF4. This indication of an excited state Xcpd radical hydrogen abstraction reaction with substituted methanes is further explored by ab initio quantum chemistry techniques at the (7×7) CASSCF/6-31G (complete active space self-consistent field) and cc-pVDZ levels for cpd–CH4. Calculations confirm the idea that the ground state cluster has a reaction barrier (approximately +170 kJ/mol) and a positive free energy of reaction (∼80 kJ/mol). The excited cpd radical, however, can react with CH4 along a barrierless path to generate substantial hot ground product states (C5H6 and CH3). Experimental data are consistent with an Xcpd–C2H4 addition reaction, as well.

Structure and dynamics from polarized parent/product photofragmentation spectroscopy

Energy-level selected nitrosyl chloride (ClNO) was rotationally polarized (aligned/oriented) and photodissociated at 355 nm. Polarization sensitive detection of the NO(2Π) product was used to measure the magnitudes and relative phases of the transition amplitudes (T-matrix elements) for the coherent excitation of continuum wavefunctions representing the dissociating molecule on the excited potential energy surface. Rotational state-populations were also measured and a smaller number of ∣NΩ) levels were found to be occupied than in non-state-selected, jet-cooled experiments. The theoretical formalism necessary for the extraction of the T-matrix elements from fully polarized experiments is also presented. Finally, hexapole focusing curves demonstrate excellent ∣JK) energy-level selection for the parent ClNO and the NO(2Π) photofragment angular distribution demonstrates for the first time dipole orientation and rotational alignment, i.e., ∣M∣ selection, of a state-selected asymmetric top by an electrostatic hexapole.

Scattering state spectroscopy of the reaction Mg*(3s3p 1P1)+CH4→MgH(v=0,1;N)+CH3

We report scattering state spectroscopic studies of the chemical quenching dynamics of Mg*(3p(1P)) by CH4. We have measured the final-state resolved action spectra for the MgH(v=1,N) reactive product channels, following excitation of the Mg*(3p)–CH4 transient bimolecular collision complex. As in earlier work on the ground vibrational state of the product, we have found a strong electronic orbital alignment effect: Reaction to the vibrationally excited product follows only on the attractive excited potential-energy surfaces in “Π-like” symmetry. For both MgH(v=0 and 1) product channels we have found that the rotational quantum state distribution is independent of laser excitation wavelength, indicating that the rotational energy partitioning is determined by exit channel dynamics. However, our results show that the product vibrational energy disposal is a function of excitation laser wavelength, suggesting that the vibrational energy partitioning is correlated with the collisional impact parameter. We have also carried out a careful search for the MgCH3 reactive product in this system, finding no evidence for any observable branching to this product. We discuss the implications of these results for the chemical dynamics of this metal-alkane reaction system.

Photodissociation of tert-Butyl Iodide at 277 and 304 nm: Evidence for Direct and Indirect Dissociation in A-Band Photolysis of Alkyl Iodide

The photodissociation of t-C4H9I has been studied at 277 and 304 nm in a supersonic molecular beam. The fragments (I and t-C4H9 radical) are selectively ionized by resonance-enhanced multiphoton ionization and then projected onto a two-dimensional position-sensitive detector to obtain their translational energy and angular distributions. The energy distribution is found to consist of three components: one Maxwell−Boltzmann and two Gaussian distributions. Their anisotropy parameters range from 0.7 to 1.6 and display a parallel transition characteristic, where the greater the kinetic energy of the component, the stronger its anisotropy. From present and previous work, these three components are interpreted in terms of three independent reaction paths on an excited potential energy surface: (1) the prompt dissociation along the C−I stretching mode for the high-energy component, (2) the repulsive mode along the C−I stretching, coupled with some bending motions for the medium-energy component, and (3) the in...

Violation of the weak noncrossing rule between totally symmetric closed-shell states in the valence-isoelectronic series O3, S3, SO2, and S2O

The ground and next excited 1A1 potential energy surfaces of the valence-isoelectronic molecules O3, S3, SO2, and S2O have been studied using full-valence-space-multiconfiguration self-consistent-field calculations. An open and a ring minimum were found for all species. In each case, interconversion between the C2v open and closed isomers proceeds via a C2v transition state which lies on a very sharp ridge separating the basins surrounding the minima. In the immediate vicinity of these transition states, the upper state for each species is found to have a minimum, also in C2v. Furthermore, not far away on the C2v ridges, the 1 1A1 and 2 1A1 surfaces intersect conically for all molecules.

Chemical reaction within the electronically excited B(2s22D)–H2 complex

The reactive decay of electronically excited B(2s22D)–H2 complexes has been investigated through the detection of chemiluminescence from the formation of electronically excited BH products. Formation of BH in its A1Π and b3Σ- electronic states was detected through the observation of emission in the A1Π→X1Σ+ and b3Σ-→a3Π band systems. In previous work, the excited electronic states of the B···H2 complex were probed through fluorescence depletion spectroscopy. The fluorescence depletion spectrum extends to energies both higher and lower than the excitation energy to the B(2s22D)+H2 asymptote, suggesting that there are both attractive and repulsive excited BH2 potential energy surfaces in the Franck–Condon region. BH chemiluminescence is observed most strongly with excitation into the lower-energy portion of the electronic transition. This implies that reaction to yield BH(A1Π, b3Σ-) occurs mainly on attractive excited BH2 surfaces. The chemiluminescence spectra have been compared with simulations in order to deduce the degree of vibrational and rotational excitation of the BH products. The products are found to have considerable internal excitation, which depends on the excitation wavenumber. All bound BH(A1Π) and all energetically allowed BH(b3Σ-) rovibrational levels were found to be significantly populated. These observations are employed to draw inferences on the reaction dynamics occurring on these excited BH2 potential energy surfaces. The reactive nature of the B(2s2 2D)–H2 surfaces is contrasted with the apparent nonreactivity of the B(23s2S)–H2 surface.

Towards a quantitative characterization photoisomerization rates

Abstract